This invention relates to a tube manifold of the type that directs the flow of a heat transfer fluid to and from a heat exchanger immersed in another heat transfer fluid and, more particularly, to a combined inlet/outlet header for directing the flow of steam through a superheater disposed in the flue gas stream.
It is often necessary to transfer heat between two fluids without the two fluids coming into contact. This is frequently done by submerging a heat exchanger in a first heat transfer fluid and passing a second heat transfer fluid through the heat exchanger, such that the fluids pass along opposite sides of the heating surface and heat is transferred between the two fluids across the heating surface. A conventional heat exchanger of this type comprises a shell adapted to confine the first heat transfer fluid, a heat exchanger typically in the form of a bundle of tubes submerged in the first heat transfer fluid, and a tube manifold for directing the flow of the second heat transfer fluid through the shell to the heat exchanger, through the heat exchanger and back out of the shell.
A major problem is encountered when utilizing such a heat exchange arrangement if there is a substantial difference in temperature between the two heat transfer fluids or if the second heat transfer fluid undergoes a substantial temperature change as it passes through the heat exchanger resulting in a sizable temperature gradient within the tube manifold itself. Temperature differences such as these result in thermal stresses between the shell that confines the first heat transfer fluid and the tube manifold where it penetrates the shell, and also within the tube manifold itself.
If there is a substantial temperature difference between the shell and the tube manifold, the resulting thermal stresses could cause rupture of the weld joining the tube manifold to the shell and destroy the seal therebetween. In practice, this problem is typically avoided by incorporating a nozzle into the shell and a thermal sleeve in the tube manifold at the point where the tube manifold passes through the nozzle and out of the shell.
If substantial temperature differences exist within the tube manifold, the resulting thermal stresses could result in rupture of the tube manifold and destroy the required separation between the inlet and outlet fluid pathways through which the second heat transfer fluid passes. This difficulty is most often avoided by providing a flexible section of piping within the tube manifold to absorb the differential expansion between the inlet and outlet pathways or by providing a fluid-tight slip joint arrangement at the point where the inlet and outlet pathways contact so as to permit differential expansion therebetween.
Accordingly, it is the purpose of this invention to provide a heat exchanger incorporating a tube manifold of unique design, thereby eliminating the aforementioned thermal stress problems and providing a simplified construction.